Condensation problems in cool roofs

In some regions of the United States so called cool roofs have become mandatory in order to save cooling energy in summer and it is expected that these roofs will also become more widespread in other parts of the world. A cool roof uses a bright surface to reflect incident solar radiation which significantly lowers the day-time surface temperature compared to conventional roofs with bituminous felt. However, since most energy savings measures involve some sort of moisture related issue, the question is whether the widespread application of these roofs may lead to durability problems. There are already rumors that the so-called self-drying roofs that do not have a vapor barrier might face moisture accumulation when equipped with a reflective surface, because the solar vapor drive helping to dry out the roofs during summer time is diminished. In order to clarify this important durability issue experimentally verified hygrothermal simulations have been carried out on light-weight flat roofs with and without reflective surface layer. Because the long-wave radiation to the sky is an important factor for the night-time roof temperature and thus also for the risk of interstitial condensation, the sky radiation has been measured as part of the meteorological data collection at the field test site in Holzkirchen. Together with continuous surface temperature recordings of different roofs these meteorological data have been used to validate the new radiation exchange model of a hygrothermal simulation tool. Afterwards a typical light-weight cool roof has been selected and its moisture behavior simulated under different outdoor conditions. The results show that severe moisture accumulation will only occur in colder regions of Europe and North-America. However, among these regions there are also some where cool roofs could be beneficial for cooling energy savings.